Experimental discussion on maximum surface charge density of fine particles sustainable in normal atmosphere

Abstract

The maximum surface charge density for large spherical particles in normal atmosphere is thought to be 27 μC/m 2, assuming that the electric breakdown field strength of air is 3 MV/m. For a given surface charge density, however, the layer of a very high electric field strength for electron avalanche in air around the particle becomes thin, when the particle size is small. As a result, a higher limit in the surface charge density is expected for small particles. In the present experiment, droplets with high surface charge density in the size range between 1 and 6 μm in diameter are prepared by evaporating negatively charged liquid particles of 45 μm. For a few kinds of liquids, the size and the charge of individual droplets are measured simultaneously by using an LDV system to obtain frequency histograms for the charges of droplets with several narrow size ranges. The results indicate that the maximum surface charge density sustainable in atmosphere for negatively charged particles depends mainly on the particle size and is inversely proportional to the square root of particle size to give very high surface charge density in micrometer order size range.